Recently, miniaturization of wirings has progressed along with miniaturization of semiconductor devices. As a result, the problem that the RC delay caused by the increase in a wiring resistance and the increase in a coupling capacitance between wirings obstructs the high-speed operation of a device has become obvious. For this reason, in recent years, copper (Cu) having a lower bulk resistance than aluminum (Al) or tungsten (W), which is conventionally used, is used as a wiring material, and a low dielectric constant film (low-k film) is used as an interlayer insulating film.
However, as miniaturization further progresses, new problems are emerging in a Cu wiring. That is, according to the ITRS roadmap, the wiring width used in the 14 nm generation device is 32 nm, which is narrower than the mean free path of electrons in a Cu material, about 39 nm. Thus, an increase in a resistance value occurs due to scattering. Specifically, the resistance value of a wiring is expressed as the sum of a bulk resistance value, a resistance factor due to surface scattering and a resistance factor due to grain boundary scattering. Both the resistance factor due to surface scattering and the resistance factor due to grain boundary scattering are proportional to the mean free path of electrons. Therefore, when the mean free path of electrons becomes larger than the wiring width, the collision of electrons with a wiring side surface or a grain boundary becomes dominant, and the resistance value increases due to scattering. This becomes conspicuous as the wiring becomes finer.
Therefore, as a wiring material, ruthenium (Ru) in which a bulk resistance value is not as low as that of Cu and in which a mean free path of electrons within the material is shorter than that of Cu has been studied. Specifically, the bulk resistance value of Ru is 7.1 μΩ-cm, which is higher than that of Cu, 1.7 μΩ-cm. However, the mean free path of electrons of Ru is 10.8 nm, which is shorter than that of Cu, 38.7 nm.
In addition, the melting point of Ru is 2334 degrees C., which is higher than the melting point of Cu, 1085 degrees C. Therefore, Ru is more advantageous than Cu in terms of electro-migration resistance.
Unlike Cu, Ru hardly diffuses into an insulating film. Thus, a barrier property is not required in a base film of a Ru film. However, it is difficult to directly form a Ru film on an insulating film with good adhesion. For this reason, there has been proposed a technique in which a TiN film as a base film is formed on an insulating film and a Ru film is formed on the TiN film to form a Ru wiring.
On the other hand, as a technology for forming a Cu wiring, there is known a technology in which a barrier film is formed on an interlayer insulating film having a trench on the surface of a semiconductor wafer and a Cu film is embedded in the trench and then planarized by a CMP (Chemical Mechanical Polishing) method. Therefore, even when forming a Ru wiring, it is conceivable that after a Ru film is formed it is then planarized by a CMP process. Although not an example of a wiring, it is known that an accumulated anode electrode (SN) is formed by depositing a Ru film and then performing a planarization process by a CMP method or the like.
However, when a TiN film as a base film is formed on an insulating film and a Ru film is formed on the TiN film, since a tensile stress acts on both the TiN film and the Ru film, the stress becomes larger due to the laminated structure. For this reason, a wiring is subjected to a great stress. If the stress of the wiring is large, there is a concern that deformation of a wiring pattern such as collapse or undulation occurs. In particular, when a wiring structure becomes finer, an interval between wirings becomes shorter so that deformation of the wirings is more likely to occur.
In addition, Ru is a noble metal and has a low ionization tendency. Therefore, it is difficult to remove a Ru film on the surface of a semiconductor wafer by CMP. When CMP is used for planarization performed after embedding a Ru film in a trench, there could be a problem in that a lot of time is taken.